The present invention relates to a modulation and demodulation method and device with adjustable protection level.
It applies particularly to the construction of receiver-transmitters for narrow band digital radio-telephony and it relates more particularly to frequency or phase modulation systems of the 4FSK type with progressive transition in which the signalling element, also called the symbol, conveys two bits of information or dibits; FSK being the abbreviation for "Frequency Shift Keying".
In known modulation systems of 4FSK type, the signalling elements or symbols are organized in the form of two bits, called a dibit, with which modulation frequencies are associated. For example, with the dibit (0, 0) is associated a frequency Fo, with dibit (0, 1) is associated a frequency F1, with dibit (1, 1) is associated a frequency F2 and with dibit (1, 0) is associated a frequency F3, these frequencies being all different from each other.
The correspondence between dibit and transmitted frequency is determined so that there is only one difference bit between the dibits associated with two adjacent frequencies so as to make the error rate per bit minimum at reception.
Nevertheless, the presence of noise which normally accompanies each dibit generally affects this type of transmission, all the more so since the connections between transmitter and receivers are heavily loaded with noise. Under these circumstances, the risks of confusion between frequencies which are adjacent in the modulation diagram are increased and the quality of restoration of the original messages by the demodulator is affected.
For some transmissions, as is the case for example for the digital transmission of speech, confusion between adjacent frequencies is not troublesome for it minimally alters the restoration of the signal, and a relatively high error rate may further be accepted without that adversely affecting the intelligibility of the speech at reception.
On the other hand, for other transmissions, it is imperative for the restoration of the messages to be, if not perfect, at least more faithful. That is the case for example for transmitting the reference amplitude in speech transmission, for any alteration results in considerable fluctuations of the synthetic speech level obtained which adversely effects understanding. The fragility of the corresponding binary fields requires them to be accompanied by redundancy bits or error self correction code words, of BCH type for example, where BCH designates the codes known under the name Bose-Chaudhuri-Hocquenghem.
However, these codes, have possibilities which are often limited to the correction and detection of a reduced number of errors, and above a given number of errors the transmission must be rerun.